This invention relates to a control strategy for a motor of an electric power assisted steering system.
It is known to provide an electrically power assisted steering system in which an electric motor applies an assistance torque to a part of a steering system to make it easier for the driver to turn the wheels of the vehicle. The magnitude of the assistance torque is determined according to a control algorithm which receives as an input one or more parameters such as the torque applied to the steering column by the driver turning the wheel, the vehicle speed and so on.
For accurate control of the motor torque it is essential to have control over the current applied to the motor. Typically a star connected three phase motor operated according to a Pulse width modulation control/drive strategy is used, each phase being connected to upper and lower drive stage switches connected to the battery supply and an earth respectively. In a PWM strategy each phase is driven with a cyclic PWM drive signal having a first state and a second state and a duty ratio indicative of the ratio of the time spent in each state in a cycle. The torque required from the motor is determined by a control circuit in terms of d-q axis motor current demand signals. These are then converted as required by a drive circuit into three phase currents in the static frame of reference, which requires knowledge of the motor rotor electrical angle of position. A position sensor may be provided that measures the rotor position or the system may be of the sensorless type, such as that taught in WO 2004/023639. Finally, using measurements of the actual current as feedback the pulse width modulation (PWM) duty cycles for each phase that are needed to produce the required actual average currents are calculated and used to drive the motor phases.
The motor draws current from the vehicles electrical supply, typically a battery which is topped up by an alternator that is driven by the drive train of the vehicle (either taking power from the engine or regenerative power during braking). The current drawn by the motor is a function of the battery voltage and the duty ratio of the drive signal applied to each phase.
At times where high assistance is required, the duty ratio of the switches will be high and the overall current drawn by the motor from the battery will in turn be high. For a healthy vehicle electrical system, the high current demand can generally be met by the alternator so the battery does not become depleted. The maximum current draw of the motor should be set to a level which can be met by the alternator. In the case where a sudden change from low assistance demand to high assistance demand occurs the alternator may not be able immediately to provide the current needed in which case some of the current will be drained from the battery until the alternator has had time to ramp up. If the battery is partially or fully depleted, or perhaps disconnected, the sudden increased demand for current may not be met, resulting in a drop in voltage until the alternator, which only reacts to changes in battery voltage. This typically presents itself to the driver of the vehicle by dimming of the lights.